1. Field of the Invention
The present invention relates to an image forming method and apparatus for producing a fluid having a predetermined density and/or a predetermined color by changing a mixture proportion of a plurality of recording liquids (inks) based on an image signal and leading the fluid to an image receiving medium to form an image.
2. Description of the Prior Art
Unexamined Japanese Patent Publication (KOKAI) No. 232440/1995 (corresponding to U.S. Pat. No. 5,841,448) discloses an ink-jet recording head, in which an optical sensor is disposed in the recording head, and ink non-ejection or defective printing is prevented by detecting presence/absence of an ink in an ink channel. This also discloses that two electrodes are disposed in an ink tank, and the presence/absence of the ink in the ink tank is detected from a change of electric resistance between the electrodes.
Moreover, a method of changing a mixture proportion of a plurality of inks to change density or color, and forming an image has already been proposed. For example, U.S. Pat. No. 4,109,282 discloses a printer in which a valve called a flap valve is disposed in a flow channel for leading two liquids, that is, clear ink and black ink onto a substrate for forming an image. The flow channel of each ink is opened/closed by displacing this valve so that two liquids are mixed in a desired density and transported onto the substrate. This enables printout of an image having the same gray scale information as that of image information displayed on a TV screen. In this reference, it is disclosed that a voltage is applied between the flap valve and an electrode disposed on a surface opposite to the flap valve and the valve itself is mechanically deformed by an electrostatic attracting force to cause displacement of the valve. The ink is absorbed by a capillary phenomenon between fibers of a print paper.
U.S. Pat. No. 4,614,953 discloses an ink-jet printer head apparatus by which only a desired amount of a plurality of inks having different colors and solvent is led to a third chamber and mixed therein. In this reference is disclosed that a chamber and a diaphragm-type piezoelectric effect device attached to this chamber are used as means for check-weighing the desired amount of ink and a pressure pulse obtained by driving this piezoelectric device is used.
Unexamined Japanese Patent Publication (KOKAI) No. 201024/1993 discloses an ink jet print head including: a liquid chamber filled with a carrier liquid; ink jet driving means provided in the liquid chamber; a nozzle connected to the liquid chamber; and a mixer for mixing the carrier liquid in this nozzle with the ink. In this reference is also disclosed that adjusting means for adjusting a mixture amount of ink to provide a desired value is provided.
Similarly, Unexamined Japanese Patent Publication (KOKAI) No. 125259/1995 discloses an ink jet recording head including: first and second supply means for supplying inks having first and second densities, respectively; and control means for controlling a supply amount of the second ink by the second supply means so that a desired ink density can be obtained.
In this reference, a micro-pump which has an exclusive heating device and is driven by its heat energy is disclosed as the control means. As this micro-pump, there is disclosed an example such that the heat energy is generated by the heating device and a pressure obtained by nucleate boiling caused due to the heat energy is used to drive, for example, a piston-type valve or a cantilever-like valve. Further, this reference describes that an ink inflow can effectively be controlled in an area where the inflow is particularly small by adopting an actuator consisting of shape memory alloy for use in this valve.
Unexamined Japanese Patent Publication (KOKAI) No. 207664/1991 discloses an ink jet printer having a structure similar to that in the above-mentioned U.S. Pat. No. 4,614,953, but does not use a third chamber for mixing a plurality of inks.
Unexamined Japanese Patent Publication (KOKAI) No. 156131/1997 discloses an ink jet printer comprising a plurality of printer heads for forming an image having multiple colors based on image data. Ink and diluent are mixed at a predetermined mixture ratio to obtain a diluent ink, which is jetted from a nozzle so that a recording image is formed on a recording medium. The ink jet printer ejects the diluent from at least one printer head out of the plurality of printer heads when all-white image data, that is, data representing that mixture amount of ink is too small to realize a clear printing density, is inputted to the plurality of printer heads. As a result, a rapid tone change (a tone jump) is prevented and the additional consumption of the diluent is suppressed to improve drying characteristics.
As described above, various systems of mixing a plurality of recording liquids (inks) have been proposed, but in this case ejection amounts of respective recording liquids is are strongly influenced by a viscosity change with a temperature change, an atmospheric pressure change, and the like. Therefore, it has been difficult to accurately obtain target values of density and color of the mixed liquid. In the apparatus disclosed in the above mentioned U.S. Pat. No. 5,841,448, it is possible to detect non-ejection or printing defect from the presence/absence of each recording liquid or the mixed liquid. However, there is a problem that only the presence/absence of the respective recording liquids is detected and that a subtle fluctuation in the mixture proportion of the respective recording liquids cannot be detected.
The present invention has been accomplished under the aforementioned circumstances, and a first object of the present invention is to provide an image forming method of mixing a plurality of recording liquids to generate a mixed liquid with a desired density and/or color, and transporting the mixed liquid to an image receiving medium to form an image, so that a mixture proportion is prevented from fluctuating by influences of recording liquid temperature, atmospheric pressure, and the like and image quality can be enhanced. A second object is to provide an image forming apparatus for direct use in carrying out the method.
According to the present invention, the first object is attained by an image forming method for ejecting a mixed liquid constituted by a plurality of recording liquids from an ejection port while changing supply amounts of the respective recording liquids based on an image signal, and transporting the mixed liquid to an image receiving medium to form an image, said method comprising steps of:
determining a target value of a mixture proportion of said plurality of recording liquids based on said image signal;
controlling the supply amounts of the respective recording liquids in such a manner that the mixture proportion of the recording liquids agrees with the target value;
detecting an actual value of the mixture proportion in the vicinity of a confluent position of said plurality of recording liquids; and
comparing the detected actual value with said target value to perform a feedback control of the supply amounts of the respective recording liquids in such a manner that there is no difference between both values.
In this case, the actual value of the mixture proportion can be detected in the vicinity of a position where the recording liquids are combined (confluent point or position), for example, in the vicinity of a right downstream portion. Specifically, the actual value is detected in a range of a channel length of the mixed liquid capacity for one pixel to a downstream side from the confluent point. The recording liquid can usually be regarded as a non-compressive fluid. Therefore, by detecting the mixture proportion from the confluent point until each recording liquid for one pixel is completely supplied, the supply amount of each recording liquid for one pixel can be fed back in real time. Therefore, the actual mixture proportion for each pixel can exactly be controlled.
The mixture proportion of the respective recording liquids can be detected based on an optical density, electric resistance, electrostatic capacity, and the like of the mixed liquid. In this case, it is assumed that the respective liquids different in density, electric resistance, and permittivity are mixed. For example, two liquids are preferably mixed.
The actual value of the mixture proportion may be estimated by detecting an actual supply amount of each recording liquid. In this case, by separately detecting the supply amounts (flow rate, flow velocity) of the respective recording liquids on an upstream side from the confluent point of the plurality of recording liquids, and obtaining a volumetric ratio, i.e., mixture proportion of the recording liquids, a result can be regarded as the actual value. The supply amount of each recording liquid can be obtained by optically detecting displacement of a movable member for controlling the supply amount, or can be detected from the electrostatic capacity which changes by the displacement of the movable member. Moreover, the supply amount can be detected by a pressure change in a recording liquid channel, or by a thermal flow rate measurement system.
The supply amount of the recording liquid is preferably corrected by a recording liquid temperature. The recording liquid temperature can be detected by a thermistor whose electric resistance changes by temperature or another temperature sensor or a thermocouple.
The supply amount of the recording liquid can also be detected by the displacement of the movable member disposed in flow rate control means of the recording liquid. The displacement of the movable member can optically be detected or can be detected by an electrostatic capacity change. By detecting environmental states in the vicinity of the ejection port of the mixed liquid, such as outside air temperature and atmospheric pressure, and the actual value or the target value of the mixture proportion is corrected. Based on the corrected actual or target value, the supply amounts of the respective recording liquids is corrected, thereby detection precision can further be enhanced.
By transporting or transferring the mixed liquid as a continuous fluid flow to the image receiving medium from the ejection port, the image can be formed (continuous coating mode). Moreover, the mixed liquid can also be transported or flied as a liquid droplet to the image receiving medium. In these cases, the mixed liquid may directly be transported to the image receiving medium from the ejection port, or transported to a final image receiving medium via an intermediate image receiving medium.
The second object of the present invention is attained by an image forming apparatus for ejecting a mixed liquid constituted by a plurality of recording liquids from an ejection port while changing supply amounts of the respective recording liquids based on an image signal, and transporting the mixed liquid to an image receiving medium to form an image, said apparatus comprising:
recording liquid flow rate control means for individually controlling the supply amounts of the respective recording liquids;
actual value detecting means for detecting an actual value of a mixture proportion of the plurality of recording liquids;
a target value processor for obtaining a target value of the mixture proportion of the respective recording liquids in accordance with the image signal;
a supply amount controller for determining the supply amounts of the respective recording liquids in such a manner that the actual value of said mixture proportion agrees with said target value; and
a driver for driving said recording liquid flow rate control means based on an output of said supply amount controller.
The actual value-detecting means can be constituted in such a manner that the actual value of the mixture proportion is obtained by calculation based on an optical density detected by a density sensor disposed in a mixed liquid channel. Instead of detecting the optical density of the mixed liquid, the actual value of the mixture proportion may be obtained by calculation in accordance with the electric resistance change, or the electrostatic capacity change of the mixed liquid.
The actual value detecting means can be constituted to detect the supply amounts of the respective recording liquids on the upstream side from the confluent point of the plurality of recording liquids, and obtain the mixture proportion from the result. In this case, as a sensor for detecting the supply amount, an optical sensor, a sensor for detecting an internal pressure change, a sensor by a thermal flow rate measurement system, and the like can be used. Moreover, the sensor may optically detect the displacement of the movable member disposed in an actuator of the recording liquid flow rate control means, or detect the electrostatic capacity change with the displacement of the movable member.
In order to control the supply amount or flow rate of the recording liquid, for example, a diaphragm-type flow control valve driven by a piezoelectric device, an electrostatic attraction force, an electrostatic repulsive force, or the like may be disposed in a plurality of recording liquid channels. In this case, a recording liquid supply pressure to the recording liquid channel is, of course, always kept to be constant. Additionally, a discharge amount of the feed pump for supplying the recording liquid to the recording liquid channel can be controlled, without using the flow control valve. Preferably, such pump is of a volumetric capacity type, and driven by a pulse motor. Instead of the pulse motor, the recording liquid feed pump may be formed by a piezoelectric device and a check valve. In this case, the driving may be performed by the electrostatic attraction force or the electrostatic repulsive force instead of the piezoelectric device.
The ejection ports for ejecting mixed recording liquids can be disposed for respective pixels arranged in a width direction of the image receiving medium, and can independently be disposed opposite to the image receiving medium. The mixed liquid droplet can be transported to the image receiving medium by an ink jet mode. Moreover, the image receiving medium may be coated with the mixed liquid by the continuous coating mode. In the continuous coating mode, the fluid (mixed liquid) ejected or extruded from the ejection port of each mixed liquid can be led to the image receiving medium through a slot opening which is elongated in a width direction of the image receiving medium. By using the slot opening is this manner, a flow of the liquid can be further stabilized as a steady flow to be led to the image receiving medium.
In the ink jet mode or the continuous coating mode, the liquid ejected from the mixed liquid ejection port can be transported to the intermediate image receiving medium such as a transfer drum, and the liquid can be further transported from the intermediate image receiving medium onto the final image receiving medium such as recording or print paper. As described above, the mixed liquid ejected from the mixed liquid ejection port can be smoothly transferred by using the intermediate image receiving medium, and deteriorated image quality due to the unevenness of the image receiving medium (final image receiving medium) such as print paper can be prevented from occurring.
In the image forming method and apparatus of the present invention, the actual value of the mixture proportion of the mixed liquid is constantly monitored in the vicinity of the confluent point of the recording liquids (in the vicinity of the downstream side of the confluent point or on the upstream side), and is compared with the target value of the mixture proportion obtained based on the image signal. And the supply amount of each recording liquid is subjected to feedback control in such a manner that the detected actual value agrees with the target value. While the recording liquids for forming the mixed liquid necessary for forming one pixel are supplied from the respective recording liquid channels, the supply amounts of the respective recording liquids are controlled to be corrected for the same pixel in real time. Therefore, the mixture proportion can always accurately be controlled, and the image quality is enhanced.
In the present invention, the image formed on the image receiving medium includes graphical intelligence patterns such as alphanumeric characters, graphical display, line art, and other image information.